A Gain-Adaptive Column Amplifier for Wide-Dynamic-Range CMOS Image Sensors

A robust gain-adaptive column amplifier scheme, which is friendly to the digital correlated multiple sampling (DCMS) A/D conversion is proposed to extend the dynamic range of CMOS image sensors. It lowers the noise at low light levels and relaxes the gain to prevent saturation at high light levels, while a low-noise readout circuit is not necessary because of the dominance of photon shot noise. Since the difference between the reset and the signal levels at a 4-T pixel output is compared with reference voltages to detect the suitable gain values, the fixed pattern noise (FPN) caused by this gain-detection scheme is estimated to be 20 times lower than that of the work in which only the pixel signal level is compared. Operation analysis and Monte Carlo simulations show the immunity of the circuit to unwanted offsets. Noise analysis and SNR calculation show that the FPN by the gain-detection scheme is more suppressed with either more gain options or the DCMS conversion applied at the amplifier output. A column-level design with a size of 9 x 500 mu m and a current consumption of 12 mu A is discussed to demonstrate the feasibility of the idea.